Wireless communications have become a focus of world-wide commercial activity. The goals for the next-generation mobile radio system include providing a wide variety of integrated multimedia services and increasing system capacity. Compared to today's cellular systems, the next-generation mobile or fixed wireless system will have a much higher channel bit rate capability to provide multimedia services to multiple users within each coverage area.
As populations grow and cities develop it becomes necessary to increase the number of cells serving a geographical user area. This includes erecting new antennas and associated infrastructure, which is expensive.
Currently, there is an extensive optical fiber infrastructure installed by the cable television industry. The optical fiber system used for cable television is configured in what is known as the "fiber backbone", as shown in FIG. 1. The fiber backbone 20 includes one or more headends ("HE") 22, which are separated by about 15 km. One or more nodes ("N") 24, 26 are connected to each headend 22 by optical fiber 28, 29. One or more subsidiary sites ("SS") 30, 32 are connected to each node 24, 26 by coaxial cable 34, 35. The subsidiary sites 30, 32 can be, for example, cable television companies or home users.
In practice, video and audio programming is received at the headend 22 (e.g., from a satellite or transmitter). Sixty to one-hundred twenty amplitude modulated vestigial sideband channels of programming are incorporated into a cable television signal. The cable television signal directly modulates a laser source at the headend 22 which generates a laser signal. The laser signal from the headend 22 is coupled into one or more optical fibers 28, 29.
Each node 24, 26 receives the laser signal from an optical fiber 28, 29. The laser signals received at the nodes 24, 26 are converted to electrical form and transmission to a subsidiary site 30, 32 (e.g., a home) is accomplished using the pre-existing coaxial wires 34, 35 and electronic amplifiers (not depicted).
Amplitude modulated vestigial sideband (AM-VSB) transmission over optical fiber is a widely accepted practice in the cable television industry. Given the dominance of cable television in delivering broadband connections to homes in the United States, attention has been given to hybrid transmission systems in which digital signals are transmitted along with the AM video channels. Hybrid systems allow for increased communication services over the existing cable television fiber backbone network.
A previous hybrid transmission approach is to add digital carriers in the unused bandwidth beyond the highest AM-VSB channel. Unfortunately, while the standard analog laser used for CATV transmission can easily accommodate the increased modulation bandwidth, the coaxial wires and electronic amplifiers may not be able to handle the higher frequencies encountered when this previous approach is used.